Unless it's continuously reapplied, sunscreen can actually attack the skin and leave it vulnerable to ultraviolet (UV) radiation, concludes a University of California, Riverside study.

The researchers found that, over time, molecules in sunscreen that block UV radiation can penetrate into the skin and leave the outer layer susceptible to UV, CBC News reported.

The study appears in an upcoming issue of the journal Free Radical Biology & Medicine.

"Sunscreens do an excellent job protecting against sunburn when used correctly," Kerry Hanson, a research scientist in the university's department of chemistry, said in a prepared statement.

"This means using a sunscreen with a high sun protection factor and applying it uniformly on the skin. Our data show, however, that if coverage at the skin surface is low, the UV filters in sunscreens that have penetrated into the epidermis can potentially do more harm than good," he said.